Generally speaking, in the field of optical fibers, the concept of effective area may be used for calculating non-linear effects. An optical fiber's effective area usually corresponds to the usable portion of the optical fiber, which is defined based upon the modal distribution of the field propagating in the optical fiber. By way of non-limiting example, the effective area (Aeff) is defined as follows:
            A      eff        =          2      ⁢      π      ⁢                          ⁢                                    (                                          ∫                0                ∞                            ⁢                                                                                                              F                      ⁡                                              (                        r                        )                                                                                                  2                                ⁢                r                ⁢                                  ⅆ                  r                                                      )                    2                                      ∫            0            ∞                    ⁢                                                                                      F                  ⁡                                      (                    r                    )                                                                              4                        ⁢            r            ⁢                          ⅆ              r                                            ,where F(r) is the modal distribution of the fundamental mode (LP01) at the radius r (i.e., at the polar distance r in the polar coordinates of a point in a axis system transverse to and centered relative to the optical fiber).
Moreover, for optical fibers, the refractive-index profile is generally classified according to the graphical appearance of the function associating the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding (e.g., an outer optical cladding) is shown on the y axis. The outer cladding, functioning as an optical cladding, typically has a refractive index that is substantially constant. This outer cladding is typically made of pure silica but may also contain one or more dopants.
The refractive-index profile is referred to as a “step” profile, “trapezoidal” profile, “alpha” profile, or “triangular” profile for graphs having the respective shapes of a step, a trapezoid, an alpha, or a triangle. These curves are generally representative of the optical fiber's theoretical profile (i.e., the set profile). Constraints in the manufacture of the optical fiber, however, may yield a slightly different refractive-index profile.
An optical fiber (i.e., a glass fiber typically surrounded by one or more coating layers) conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core nc is typically greater than the refractive index of the optical cladding ng (i.e., nc>ng). As will be understood by those having ordinary skill in the art, the propagation of an optical signal in a single-mode optical fiber includes a fundamental mode, typically denoted LP01, which is guided in the core, and secondary modes, which are guided over a certain distance in the core and the optical cladding.
Optical fibers typically include a central core that has a radius r1 and a refractive index n1>ng. Optical fibers may also include one or more cladding layers positioned between the central core and the optical cladding. These cladding layers have respective radii ri and refractive indices ni<n1 that may be less than or greater than the refractive index ng of the optical cladding. A cladding layer between the central core and the optical cladding with a refractive index similar to the optical cladding's refractive index ng is sometimes referred to as an internal cladding. Additionally, a cladding layer with a refractive index lower than the optical cladding's refractive index ng is sometimes referred to as a buried cladding, trench, and/or buried trench.
Single-mode optical fibers (SMFs) with step-index profile are often used within optical-fiber transmission systems as line fibers. Such optical fibers typically possess values for chromatic dispersion, chromatic-dispersion slope, cut-off wavelength, and effective area that comply with specific telecommunications standards.
The cable cut-off wavelength is conventionally measured as being the wavelength at which the optical signal is no longer single mode after propagating over 22 meters in the optical fiber, as defined by subcommittee 86A of the International Electrotechnical Commission (IEC) in standard IEC 60793-1-44. The IEC 60793-1-44 is hereby incorporated by reference in its entirety.
In most circumstances, the secondary mode that best withstands bending losses is the LP11 mode. The cable cut-off wavelength is thus the wavelength from which the LP11 mode is sufficiently attenuated after propagating for 22 meters in an optical fiber. The method proposed by the IEC 60793-1-44 standard considers that the optical signal is single mode as long as the attenuation of the LP11 mode is greater than or equal to 19.3 decibels (dB). According to the recommendations of IEC subcommittee 86A in standard IEC 60793-1-44, the cable cut-off wavelength is determined by arranging the optical fiber such that it forms two loops having a radius of 40 millimeters (mm) and arranging the remainder of the optical fiber (i.e., 21.5 meters of optical fiber) on a mandrel having a radius of 140 millimeters.
The IEC 60793-1-44 standard also defines the effective fiber cut-off wavelength. The effective fiber cut-off wavelength is conventionally measured as the wavelength at which the optical signal is no longer single mode (i.e., when the attenuation of the LP11 mode is greater than or equal to 19.3 dB) after propagating over two meters of fiber, while arranging one loop of the optical fiber on a mandrel having a radius of 140 millimeters.
Typically, for terrestrial transmission systems, standard single-mode fibers (SSMF) are used. Such standard single-mode fibers have a positive dispersion (D) and a positive dispersion slope (P), an effective area (S) of about 80 μm2, and an attenuation of about 0.19 dB/km (measured at a wavelength of 1550 nm).
Submarine transmission systems with repeaters typically use hybrid transmission lines with both (i) optical fibers having a positive dispersion, a large effective area (about 100-110 μm2), and a low attenuation (0.17-0.19 dB/km measured at a wavelength of 1550 nm) and (ii) optical fibers with negative dispersion.
Undersea transmission systems without repeaters typically use transmission lines including combinations of optical fibers having a positive dispersion (e.g., pure-silica-core fibers having an effective area of between 80 μm2 and 110 μm2). Record-breaking results in terms of capacity relative to distance have recently been obtained in the laboratory with optical fibers having an effective area of about 120 μm2 using coherent detection and sophisticated modulation formats.
As known by those having ordinary skill in the art, an increase in the effective area of a transmission optical fiber contributes to the reduction of non-linear effects in the optical fiber. A transmission optical fiber having an enlarged effective area facilitates transmission over a longer distance and/or an increase in the functional bands of the transmission system. Typically, optical-fiber modifications that are intended to achieve increased effective areas also increase bending losses and the optical fiber's cut-off wavelength.
At present, while maintaining low bending losses and a cable cut-off wavelength (λcc) less than 1450 nanometers (nm), the maximum achievable effective area (Aeff) is about 130 μm2. Furthermore, an effective area (Aeff) of about 130 μm2 currently represents the maximum effective area (Aeff) suitable for ensuring a chromatic dispersion and chromatic dispersion slope that can be processed either by compensation modules or by coherent detection and sophisticated modulation formats.
To increase the effective area of single-mode optical fibers, step-index fiber profiles are usually employed in either constant cladding or buried cladding configurations.
U.S. Pat. No. 6,658,190, which is hereby incorporated by reference in its entirety, describes optical fibers that have an effective area (Aeff) greater than 100 μm2. The examples described in U.S. Pat. No. 6,658,190, however, exhibit undesirably increased bending losses and/or fabrication costs. Sample 5 of U.S. Pat. No. 6,658,190 discloses a single-mode-fiber profile with constant cladding and an effective area of 155 μm2, but its macrobending and microbending losses are unsatisfactory. Samples 6, 7, and 8, which use step-index fiber and buried-cladding profiles, have effective areas greater than 150 μm2 but the outer radius of the buried cladding is very large (e.g., greater than 29 microns (μm)), which increases fabrication cost.
U.S. Pat. No. 7,076,139, which is hereby incorporated by reference in its entirety, describes step-index and buried-cladding optical fibers for which Δn1<4.4×10−3 and Aeff≧120 μm2. None of the examples described in U.S. Pat. No. 7,076,139, however, use a fiber profile capable of simultaneously achieving an increased effective area, low bending losses, and a low buried cladding outer radius. Table 2 in U.S. Pat. No. 7,076,139 shows an example with Aeff of 156 μm2, but the outer radius of the buried cladding is approximately 31 microns, which significantly increases manufacturing costs.
U.S. Pat. No. 6,483,975, which is hereby incorporated by reference in its entirety, discloses optical fiber profiles including a central core, an intermediate cladding, and a buried cladding having an effective area (Aeff) greater than 100 μm2. The radius of the central core, however, is too small (e.g., less than 6.4 microns) and the buried cladding is too large (e.g., greater than 15 microns) and insufficiently deep (e.g., a depth above −2.9×10−3) to make it possible to obtain both an effective area greater than 150 μm2 and satisfactory bending losses. Additionally, the outer radius of the buried cladding is very large, which significantly increases manufacturing costs.
European Patent No. 1,477,831 and its counterpart U.S. Pat. No. 6,904,218, each of which is hereby incorporated by reference in its entirety, describe optical fiber profiles including a central core, an intermediate cladding, and a buried cladding. The disclosed optical fibers have an effective area (Aeff) greater than 95 μm2 (e.g., an Aeff of about 171 μm2) and a cable cut-off wavelength (λcc) less than or equal to 1310 nanometers. With such cable-cut-off-wavelength values, however, it is not possible to achieve both an effective area (Aeff) greater than or equal to 150 μm2 and good bending loss performance. Moreover, the outer radius of the buried cladding is very large (e.g., greater than 33 microns), which significantly increases manufacturing costs.
U.S. Pat. No. 7,254,305, which is hereby incorporated by reference in its entirety, describes optical fiber profiles including a central core, an intermediate cladding, and a buried cladding. The disclosed optical fibers have a chromatic dispersion slope less than 0.07 ps/(nm2·km) and attenuation less than 0.20 dB/km. For a cable cut-off wavelength (λcc) of 1854 nanometers, the maximum effective area (Aeff) disclosed is 106 μm2. For the disclosed optical-fiber profiles, however, the values of Δn1 (i.e., the refractive index difference between the central core and the outer cladding) and Δn3 (i.e., the refractive index difference between the trench and the outer cladding) are too high to obtain both an effective area greater than 150 μm2 and satisfactory bending losses.
European Patent No. 1,978,383 and its counterpart U.S. Patent Publication No. 2011/0044595, each of which is hereby incorporated by reference in its entirety, describe optical fiber profiles including a central core, an intermediate cladding, and a buried cladding. The disclosed optical fibers have an effective area (Aeff) greater than or equal to 120 μm2 and an effective fiber cut-off wavelength less than 1600 nanometers. All the disclosed examples have central core refractive index difference Δn1 greater than 3.9×10−3. With the disclosed constraints on cut-off wavelength and central core refractive index difference, however, it is not possible to achieve an effective area greater than or equal to 150 μm2. The highest disclosed value of effective area is only 135 μm2.
International Patent Application Publication No. WO 2008/137150 and its counterpart U.S. Pat. No. 7,555,187, each of which is hereby incorporated by reference in its entirety, describe a fiber profile including a central core, an intermediate cladding, and a buried cladding. The disclosed optical fibers have an effective area (Aeff) greater than 110 μm2, a cable cut-off wavelength (λcc) less than 1500 nanometers, and macrobending losses less than 0.7 decibels per turn (dB/turn) for a bend radius of 10 millimeters at a wavelength of 1550 nanometers. With the disclosed constraints on cable cut-off wavelength (λcc) and/or bending losses, however, it is not possible to achieve an effective area greater than or equal to 150 μm2.
International Patent Application Publication No. WO 2008/137150 discloses two fiber examples with effective area greater than 150 μm2. Fiber 7 has an effective area of 155 μm2, but the outer radius of the buried cladding is too large (e.g., approximately 27.5 microns), which increases fabrication costs. Fiber 8 has an effective area of 167 μm2 and a buried-cladding outer radius of 18.5 microns, but the macrobending losses are relatively high (e.g., about 10 decibels per meter (dB/m) at a wavelength of 1550 nanometers and greater than 0.1 dB/100 turns for a bend radius of 30 millimeters at a wavelength of 1625 nanometers). Moreover, the volume of the trench (i.e., the buried cladding) does not account for the dimensions of the central core.
Therefore, a need exists for an improved bend-resistant, single-mode optical fiber having an effective area of at least 150 μm2 that can be achieved without significantly increasing manufacturing costs.